This invention pertains to protective surface coatings commonly known as paint coatings. Protective surface coatings are organic compositions applied to substrates to form continuous films which are cured or otherwise hardened to provide protection as well as a decorative appearance to the substrate. Protective surface coatings ordinarily comprise an organic polymeric binder, pigments, inert fillers and other additives. The polymeric binder functions as an organic vehicle for the pigments, inerts, and other additives in wet coating compositions and further functions as a binder for the pigments and inert fillers in the cured or hardened paint film. The thermosetting polymeric binders of this invention are thermosetting binders based on the coreactive components of a functional polyester and a coreactive amine crosslinker.
Commercial liquid coatings applied to metallic substrates such as coil steel typically consist of a linear or lightly branched polyester resin adapted to crosslink upon heating with a melamine or similar amine based crosslinking agent. The polyester resins comprise the reaction product of polyfunctional acids with glycols. The cured coatings desirably exhibit excellent hardness and solvent resistance. However, such polyesters designed to meet the hardness, flexibility and solvent resistance requirements are unstable and crystallize out of organic solvent solution in a relatively short time.
U.S. Pat. No. 4,520,188 suggests a polyester based on the combination of glycols consisting of a neopentyl glycol and 1,4-cyclohexanedimethanol copolymerized with dicarboxylic acids and crosslinked with melamine resin. The coating is said to resist crystallization. However, the polyester polymer disclosed in U.S. Pat. No. 4,520,188 is not entirely resistant to crystallization and exhibits instability over time in respect to crystallizing out of an organic solvent solution.
It now has been found that polyester coatings, particularly polyester coil coatings, can be substantially improved based on polyesters containing a non-symmetrical glycol reactant comprising 2-methyl-1,3-propane diol in conjunction with other glycols and dicarboxylic acid reactants. Use of 2-methyl-l,3-propane diol not only maintains the superior film integrity properties such as hardness, bend flexibility, and resistance to solvent degradation, but dramatically improves the resistance to crystallization or formation of polyester crystals in storage. In essence, the polyester polymer of this invention exhibits excellent chemical and physical film properties while the resin itself is essentially immune to crystallization when dissolved in an organic solvent mixture. These and other advantages will become more apparent by referring to the detailed description and the illustrative examples.